Each side of the upper jaw of an adult dog has 3 incisor teeth, 1 large canine tooth, 4 premolars and 2 molar teeth. The lower jaw has 3 incisors, 1 canine tooth, 4 premolars and 3 molars. A dog, therefore, should have 42 permanent teeth. Puppies are born with no teeth but deciduous teeth begin to appear at 3 to 4 weeks of age. In total a puppy will cut 28 "milk" teeth. At about 3 months of age, the permanent teeth will start to erupt and displace the deciduous teeth. By the time the puppy is 7 months old all permanent teeth should be present.
Terriers should have a "scissor" bite - the top teeth closely overlapping the bottom teeth.
It is vital to maintain a good standard of dental hygiene - nature's way is to give the dog bones to chew and you will never find a dog that has to hunt for his food with bad or dirty teeth. Another way is to brush the dog's teeth regularly using a canine toothpaste. Do not use human toothpaste. Make sure your dog has plenty of suitable things to chew on and this is particularly helpful to him when he is cutting his permanent teeth.
Diagram of skull is reproduced by kind permission of Trevor Turner, BVetMed,MRCVS.